Debug stabstrings

The stabstring codes are defined to specify the location of objects in VRs.

New stabstring codes are defined to specify the location of objects in VRs. A code of "X" describes a parameter passed by value in the specified vector register. A code of "x" describes a local variable residing in the specified VR. The existing storage classes of C_LSYM (local variable on stack), C_PSYM (parameter on stack) are used for vector data types in memory where the corresponding stabstrings use arrays of existing fundamental types to represent the data. The existing storage classes of C_RPSYM (parameter in register), and C_RSYM (variable in register) are used in conjunction with the new stabstring codes 'X' and 'x' respectively to represent vector data types in vector registers.